Question about measuring and estimating ppm

I'm in early to mid veg stage, and my res are both 400ppm and a little above, which is fine for me right now.

My question is about determining the right amount of nutes to add, taking into consideration my starting tap water ppm...which is a little under 200.

I know it isn't ideal using well water, but I am, and the plants are fine so far. If, for example, I'm trying to get a ppm of 500 next time I change the res, I would add 300 ppm of nutes to the already 200 ppm water, correct?

But in so doing, am I not depriving the plants of the 200ppm of nutes that well water is taking the place of? If 500ppm of nutes is what I'm going for, then wouldn't I add enough to make the water 700ppm, since 200 of that is just plain water?

If I was starting with 0ppm water and adding 500ppm of straight nutes, compared to 200ppm starting water and adding 300ppm of nutes to make 500...then I'm adding almost half the strength of nutes...just because my starting well water is 200ppm.

So is there a way to make up the lost nute total I'm to be going for? Or am I looking at this the wrong way

i prefer using RO for this very reason. i'm on well too but it's easier for me to buy 15 gals of RO at walmart once a week.

Just in case anyone else reads this...

TDS is measured as ppm but in the 500 scale. Usually when dealing with a PPM meter it's expressed in 700 scale and most feed charts will give the expected 700 scale ppm value and often the EC as well. All 3 of these #'s look different but mean the same thing.

It's almost like hopping on a scale but not knowing if it's pounds or kg. So, when using a TDS meter refer to a conversion chart to get the 700 scale value. https://www.bluelab.com/knowledge-base-files/Conductivity-Conversion-Chart

Then yes... If your water is 300ppm to start, then you add your nutes and your final reading is 650 - then your nute ppm would be 350ppm.

If you use other additives - silica, ca/mg - measure ppm after each as well and write it all down. Then you'll know exactly which component adds how much to your overall ppm. You would also want to factor those out of your final ppm value as well if you're looking for the nute's contribution to ppm.

I test ppm and pH before and after each additive and write it down. If I ever forget, I can test the solution and based on it's PPM, I'll know where I left off. Also, when you know these - you can mix to ppm instead of volume. Like - lets say that you have determined that 5ml/gal of silica gives you 50 ppm - then you can also just add it slowly until you get to 50 ppm. Same thing done in different ways. If you use a 2 part nute, then if you know what your ppm should be after adding the A part - then you can also just add it until you get to your target ppm.

There's a point where too much ionic content will start to impact the plants. But don't worry that the feed chart tells you 500ppm - with your water you'll end up around 780. A lot of ca/mg will add up to 400ppm and silica maybe 50. If you used them both then your final mix would be around 1230 ppm - but the nute's contribution would still be 500ppm. And that's ok.

Make some sense?

When posting about ppm values - convert to 700. You might get some wrong advice if someone didn't realize you were giving 500 scale #'s.

Great info. Helps me a lot. I appreciate it

it really depends on whats in the 200ppm of source water... I wouldn't focus on all that... just use the source water and mix your nutes and keep a daily log or journal of your ppm and PH

follow the rule of thumb listed below


Ec/ppm goes up, PH goes down=plants require less nutes.
Ec/ppm goes down, PH goes up=Plants require more nutes
Ec/ppm stable, PH goes up=Equilibrium=Good thang.

the reservoir changes will tell you if you should feed more or less